1. Field of the Invention
The field of the invention relates to internal combustion engines having vacuum assisted brake systems.
2. Background Art
Internal combustion engines are known that improve thermal efficiency and reduce pumping work by operating lean of stoichiometry and increasing manifold pressure. It is also known to extend lean operation by performing stratified operation where fuel is injected directly into the engine cylinder during a compression stroke. These engines are also capable of performing homogeneous operation where fuel is injected during an intake, or suction, stroke. Typically, stratified operation is limited to lean air/fuel ratios, while homogeneous operation can be both lean and rich of stoichiometry.
Vehicle brake systems are also known that use vacuum assist to increase driver braking force. In these systems, vacuum generated by engine operation provides extra force to assist driver braking. When these braking systems are combined with lean burn engines, engine operation can be controlled so that desired vacuum is supplied during braking. One approach uses a vacuum pressure sensor located in a brake booster to indicate available braking vacuum. When available vacuum falls below a predetermined value, engine air/fuel ratio is decreased toward stoichiometry and stratified operation is discontinued. Vacuum sensor degradation is determined when sensor voltage is outside a predetermined range of acceptable limits. Such a system is described in U.S. Pat. No. 5,826,559.
The inventors herein have recognized a disadvantage with the above approach. In particular, in-range sensor degradation is not addressed in the above system. For example, if the sensor indicates insufficient vacuum when sufficient vacuum is present, the engine air/fuel ratio is unnecessarily decreased and fuel economy can be degraded. In other words, the prior art does not monitor such a situation.